Recently, materials displaying the ability to be reversibly switched between two stable electrical resistance states have been proposed for use in nonvolatile memories. When incorporated in a memory cell, these materials can be toggled between higher and lower electrical resistance states by applying a pulse of electrical current (“switching current pulse”) to the materials. Subsequently, after writing to a memory cell in this way, the electrical resistance state of the given memory cell can be determined (i.e., read) by applying a sensing current pulse to the material in order to determine its electrical resistance state. The amplitude of the sensing current pulse is preferably sufficiently smaller than the amplitude of the switching current pulse so that the electrical resistance state of the material is not altered by the read operation, and the written electrical resistance state persists.
Chalcogenides are one group of materials that have been proposed for use in nonvolatile memory cells because many materials within this group manifest two stable physical states having different electrical resistances. A. Pirovano et al., Electronic Switching in Phase-Change Memories, IEEE Transactions on Electron Devices, Vol. 51, No. 3, March 2004, for example, describes such a use of chalcogenides and is incorporated herein by reference. A chalcogenide typically comprises germanium, antimony, sulfur, selenium or tellurium, or a combination therof. When incorporated into a properly configured memory cell, a chalcogenide can be forced to transition between a higher electrical resistance amorphous state and a lower electrical resistance polycrystalline state by applying a switching current pulse. Importantly, the electrical resistance state into which a chalcogenide is placed is critically dependent on the amplitude of the applied switching current pulse rather than its direction.
Another group of materials displaying two stable electrical resistance states comprises transition metal oxides. The switching characteristics of memory cells comprising transition metal oxides are described in various publications, for example, A. Beck et al., Reproducible Switching Effect in Thin Oxide Films for Memory Applications, Applied Physics Letters, Vol. 77, No. 1, July 2000, and U.S. Pat. No. 6,815,744 to Beck et al., entitled “Microelectronic Device for Storing Information with Switchable Ohmic Resistance,” both of which are also incorporated herein by reference. While the mechanism for the memory effect remains under study, the materials within this class, when incorporated into a properly configured memory cell reproducibly and reversibly undergo a transition from one stable electrical resistance state to a different stable electrical resistance state in response to an applied switching current pulse. However, unlike chalcogenides, the transition from one resistance state to the other in transition metal oxides is not critically dependent on the amplitude of the switching current pulse, but rather is dependent on its direction. In other words, a switching current pulse must have a particular direction of current flow in order to cause a transition metal oxide to transition between one electrical resistance state and its other electrical resistance state.
Notably, in the case of both chalcogenides and transition metal oxides, the ratio between the higher and lower electrical resistance states is typically between about 100:1 and 1,000:1. Moreover, the higher and lower electrical resistance states have been shown to persist for time periods on the order of months without being refreshed. Nevertheless, despite these apparent advantages, chalcogenides and transition metal oxides have heretofore only been utilized independently to form memory cells capable of storing a single bit of data at any given time. Because of strong demand for smaller, higher density memories, however, there is a need for nonvolatile memory cells capable of concurrently storing two or more bits of data.